The present invention relates to a process for forming compacts from coal particulates which does not require use of a binder and to the coal compacts produced by the process. The process of the present invention can be used to form large, cylindrically-shaped compacts from coal particles (i.e., xe2x80x9ccoal logsxe2x80x9d) so that the coal can be transported in a hydraulic coal log pipeline.
Hydraulic coal log pipelines, such as that described and shown in U.S. Pat. No. 4,946,317 (Liu et al.), may be used to transport coal log compacts over long distances through a pipeline using water as a carrier fluid. In order to fully develop coal log pipeline technology as a commercially viable means of coal transport, it is necessary to develop processes for fabricating coal compacts which possess and retain sufficient mechanical strength to withstand the lengthy exposure to high pressure water and abrasion attendant pipeline transport.
It is generally known that loose coal particles can be formed into agglomerates or compacts (e.g., briquettes as well as other shapes) by compacting or extruding a mixture of coal particles and a solid or liquid binder additive (e.g., pitch, bitumen, cement, sodium silicate, sulfite lye, cellulose, starch and synthetic resins). However, the use of binders in forming coal compacts is generally undesirable because the binders add to the expense and complexity of the process, cause increased smoking when the compact is subsequently burned and render the compact generally unpleasant to handle. As a result, binderless coal compaction and extrusion processes have been developed.
Prior art binderless processes are capable of producing coal compacts having exceptional mechanical strength. However, prior art processes are typically used to produce small coal briquettes which are not intended to be exposed to the extreme pressures and degradation forces imposed by pipeline transportation. Conventional binderless coal compacts have a moisture content much lower than the saturation moisture content of the compact and, as a result, tend to exhibit a strong affinity for water. When these compacts are exposed to water, a substantial amount of water is absorbed into the structure of the compact causing significant volumetric expansion. The water-soaked compacts suffer from an excessive loss of mechanical strength and break apart. The degradation of mechanical strength induced by water absorption is exacerbated when the compacts are immersed in high pressure water such as that used in hydraulic coal log pipelines. The ability of a compact to repel water and not break apart when exposed to a high pressure water stream is of primary importance when the compact is prepared for transport in a coal log pipeline. Thus, prior art binderless processes for making coal compacts have not proved suitable for fabricating coal logs for pipeline transport.
Among the objects of the present invention, therefore, are the provision of coal compacts and a process for making coal compacts which do not require use of a binder; the provision of durable coal compacts which retain sufficient mechanical strength after exposure to high pressure water and other degradation forces imposed by hydraulic pipeline transportation and a process for making such coal compacts; the provision of coal compacts which possess increased heating value relative to the feed coal and a process for making such coal compacts; and the provision of a process for producing coal compacts efficiently and economically.
Briefly, therefore, the present invention is directed to a process for making compacts from coal particles. The process comprises heating a particulate feed comprising substantially water-saturated coal particles to a temperature greater than about 100xc2x0 C. at a pressure sufficient to prevent water in the feed from boiling. The heated feed is compacted in a mold by applying a compressive stress to the heated feed to thereby form the compact. The compact is then cooled at a pressure sufficient to prevent water in the cooling compact from boiling.
The invention is further directed to a process for making compacts from coal particles which comprises preheating a particulate feed comprising substantially water-saturated coal particles at an ambient pressure of about 1 atmosphere to a temperature less than about 100xc2x0 C. so that water in the feed does not boil. The preheated particulate feed is then compacted by applying a compressive stress to the preheated particulate feed in a mold. The compressive stress is sufficient to prevent water in the compacted feed from boiling when the compacted feed is subsequently heated to a final compaction temperature. The compact is then formed by heating the compacted particulate feed in the mold to the final compaction temperature while maintaining the compressive stress. The final compaction temperature is greater than about 100xc2x0 C. The formed compact is cooled to an ejection temperature in the mold while maintaining sufficient compressive stress on the compact to prevent water in the compact from boiling. The ejection temperature is sufficiently low such that the compressive stress can be removed from the compact after cooling without inducing water in the compact to boil. The cooled compact is then ejected from the mold.
The invention is further directed to a coal compact comprising compacted coal particles. The compact has an original tensile strength of at least about 3.4xc3x97105 Pa (50 psi) and an original compressive strength of at least about 3.45 MPa (500 psi) after being formed and cooled to about 27xc2x0 C. Furthermore, after being immersed in a static water bath pressurized to about 3.45 MPa (500 psi) gauge for a period of about 24 hours, the compact has a tensile strength of at least about 50% of the original tensile strength and a compressive strength of at least about 50% of the original compressive strength.
Other objects and features of this invention will be in part apparent and in part pointed out hereinafter.